The present invention is directed to a motion picture film wherein a layer containing colorless, transparent ultraviolet light excitable soundtracks is provided on one side of the film. This is a unique material bearing two completely independent imaging systems, the familiar silver halide system and an electrostatic system. The soundtrack images may cover the whole or part of either the front or back of the film and are coded in digital form. More particularly, the present invention is directed to the use of a two layer image receiving system to record soundtracks on a film to produce soundtracks which are substantially colorless and transparent to visible light, but fluoresce in the visible light spectrum when exposed to ultraviolet light. The soundtracks comprise a toner imprinted onto the film by means of the electrostatic imaging system.
Generally, in the prior art it has become standard procedure to provide a magnetic or optical recording track on the edge of a film adjacent the visible image when producing sound in motion pictures. The width of the track is a limiting factor since it can only be on an area not covered by the photographic image, and thus must be very narrow due to the limited width of the film. Further, when utilizing multiple analog sound tracks on a conventional 35 mm motion picture release print, there is not sufficient space on the film to provide reasonable soundtracks which have good signal to noise ratio, frequency response and high information density. The present invention, on the other hand, provides a film and a method of using such film that admits of recording the sound on the full width of a film, and thus provides improved reproduction of the sound.
A digital sound record requires a high density of information on the film. For example, a single soundtrack designed to deliver sound at 90 db. dynamic range and 0-20 KHz frequency range will require 50,000 or more 16-bit "words" or numbers per second. This amounts to more than 800,000 bit marks per second per track, or more than 4,800,000 bit marks per second for six tracks. With auxiliary timing and positioning information, and with some redundant information to allow for correction of individual bit-error, a total of about 7,500,000 bits per second is required. The area of silver halide film currently reserved for the analog soundtrack cannot sustain this level of information recording.
It is known to use various light systems, e.g., the system shown in U.S. Pat. No. 1,928,329 to Oswald, et al. and U.S. Pat. Nos. 3,508,015 and 3,522,388 to Miller. However, these systems apparently do not recognize the possibility of recording both sound and images on the same area of the film. The patent to Oswald uses a black and white film and visible light through a lens to provide the sound system while the patents to Miller utilize light emitting diodes of varying types. The systems thus suffer from the same deficiency of good sound reproduction as is encountered in the magnetic strip or variable area analog optical type of motion picture soundtrack recording.
Further, the art sometimes accomplishes multiple sound source effects by using separate, but synchronously run, film strips or magnetic tape. These systems present serious technical problems such as maintaining sound and image synchronization between the two separately run systems, especially when the strip or tape of one of the two systems has a section removed for repair or other purposes. This film may be of the standard 16 mm, 35 mm or 70 mm size. In the present invention and use, a plurality of digital soundtracks imaged in a transparent, substantially colorless material which can be excited to fluorescence by ultraviolet light are superimposed over the visual image area. One ultraviolet soundtrack exciter source serves to energize, or cause to fluoresce, all of the soundtracks.
Because of the intrinsically limited quality of optical and magnetic analog soundtracks in standard use, the motion picture industry has been unable to effectively reproduce the detailed realism, presence and aural excitement achieved with high fidelity systems at home and at discotheques and concerts. The accuracy of sound reproduction accepted as standard on records and tapes cannot physically be contained in the analog optical track standardized 50 years ago in cramped and grainy space alongside Edison's inch-wide picture. Within this decade, given digital recording, the art of high fidelity sound reproduction will improve still further, putting the film industry in worse jeopardy of failing to provide sound of equal fidelity.
Digital coding enables complete digital sound handling, including mixing and editing, usually done on magnetic tapes, without tape hiss or noise or degradation of the sound signal accumulating through successive generations of the recording, mixing, editing, mastering procedure. With the sound signal reduced to plus/minus ("yes"/"no") bits and with parity check bits to monitor the entry of errors, the identity of successive reproductions can be assured. Thus, the present invention is further directed to a film having layers which accept such digitally coded soundtrack(s) as binary number data, permitting reconstruction with absolute precision.
The archaic analog soundtrack is a "picture" of the wave nature of sound and the detail of the analog sound information must inevitably be mixed together with the intrinsic defects of the recording medium. The distortion which is characteristic of the analog recording means and the noise imposed by the coarse silver grains of the film become inseparable from the desired high fidelity sound.
The essential difference in the digital sound record is that the integrity of the sound information exists separate and immune from the physical nature of the recording medium. It is the intent of fluorescent soundtracking to record a plurality of channels of digital sound across the photograhic image space of film as transparent and colorless fluorescent digital words. In digital sound recording, the amplitude of the sound wave is "sampled", or measured, at discrete intervals at a clocked constant repetition rate, as, for example, 50,000 samples per second to record frequencies of up to 20,000 Hz. Each sample is next converted to, for example, 16 bit digital words with one or more parity check bits. The 16 bits of each word used to record the wave amplitude of the sample (the dynamic range) can write any integer between 0 and 65,535. This is considerably more information than can be derived from the compressed amplitude spike of the present standard optical analog soundtrack record that is submerged among silver grains.
A simple and inexpensive system is required for imprinting or imaging the fluoroescent digital words of the system described above. One such system, suggested for its accuracy, simplicity and ready adaptability to digital coding, is an electrostatic imaging system. A common method for fixing the electrostatic image on a substrate is by heat fusion of a toner comprised of a polymer having a melting point lower than the substrate employed. For highest optical quality the toner image may be covered with a lacquer or polymeric overcoat which matches the visible refractive index of the toner particles. The overcoat may further function to more securely fix the digital image in place and to protect the data bits from abrading in the projector or elsewhere.
It is further necessary that the fluorescent material of the toner remain bound in the toner in order to maintain distinct markings on the film. Difficulties are encountered, however, in obtaining such a polymer toner which is also fluorescent. An ordinary brightener compound present at the required concentration may suffer fluorescence quenching or may "bleed" out of the toner particles and into the support materials. It has been suggested in the prior art to make fluorescent polymers having the fluorescent compound (brightener) covalently bound to a polymer backbone. In U.S. Pat. No. 3,193,536 it is suggested to prepare a vinyl-brightener monomer and copolymerize it into the growing backbone of a suitable majority polymer. These teachings, however, are unsuitable for preparing the compounds useful in the present invention. It is exceedingly difficult to control the distribution of the brightener residues along the polymer chain and with a high loading of the brightener, non-selective positioning along the polymer backbone leads to severe fluorescence quenching. Although the patent mentions systems loaded with up to 100 percent of vinyl-brightener, no mention is made of quenching difficulties or of strategies for avoiding them. All of the practical examples deal with brightener loadings of 0.1-0.2 percent by weight, which do not provide sufficient ultraviolet absorption and re-emission for a suitable toner for the present invention.
Further problems are encountered with this approach due to the tendency for the vinyl-brightener to self-polymerize even in the solid state and to photoinitiate polymerization at any time. Additionally, the relative reactivities in polymerization may be such as to incorporate all of the vinyl-brighteners together in the first polymer chain segments formed, or in the last chain segments. This leads to severe quenching.
A second approach to providing a fluorescent polymer suitable for the present invention is to synthesize a polymer having reactive groups suitable for binding to a selected group of the brightener molecule. Problems encountered here include gross alteration of the properties of original brightener, especially the absorption and fluorescence wavelengths and the fluorescence quantum yield.
In this approach the monomer reactive with the brightener molecule may be a material such as maleic anhydride, acrylyl chloride, or methacrylyl chloride, and, subject to reasonable relative reactivities, the whole array of ordinary vinyl monomers is available to complete the copolymer chain. Even a sparingly soluble brightener can then be slowly coupled to the completed polymer. This approach requires the scrupulous exclusion of water to avoid conversion of the reactive sites to unreactive acid functions. It also requires that one consider polymerization conditions and monomer pairs which maximize the separation of the reactive sites, and thus minimize possibilities for quenching.
It is the general consensus that liquid toner development gives the best approach to high resolution electrophotography. The suspending liquid of the toner must be moderately volatile so that it can be removed by mild heating or evaporation at the end of the process. Additionally, it must have a high electrical resistivity so as not to discharge the primary images formed on exposure or contact. Additionally, the toner material must be insoluble in the suspending liquid. The solid toner particles are charged, all positive or all negative, with respect to the liquid vehicle.